Electric Circuits

Electrostatics
1. Charge (q)
  1. elementary particles
    1. electrons
    2. protons
    3. e = ± 1.6×10^-19 C
  2. A net charge occurs if there is difference between the number of protons and electrons
  3. An object can acquire a net charge by:
    1. Friction
      1. energy supplied to the outermost electrons allows them to move from the material with the least affinity for electrons to the material with the most affinity
        Triboelectric Series
    2. Induction
      1. One charged object is held near, but does not touch, another. The charged object causes the second to become polar. The second object is then earthed to neutralise the end that is not attracted to the charged object, then the ground source is removed, followed by the original charged object, leaving the second object with a net charge.
    3. Conduction
      1. Two conductor objects are touched together, allowing electrons to flow between them, 'sharing' the charge. Both object acquire the same sign of charge
  4. Law of Conservation of Charge: The net amount of charge produced in any transfer process is zero.
  5. measured in Coulombs (C)
  6. An Electroscope can be used to detect the presence of a net electric charge
2. Law of electrostatics: Like charges repel and unlike charges attract
3. Coulomb's Law: F=(kqQ)/d^2
  1. When charged objects are brought in close proximity, there is a force exerted between them. This force can be either attractive or repulsive (depending on the nature of the charges in question) and the direction of the force is along a line joining the centres of the charges. Both charges exert equal and opposite force force on the other.
  2. VECTOR
    1. Use vector addition techniques (head to tail, resolve into components) to find the resultant force when two or more charges are being considered.
4. ELectric Field (E)
  1. Region in which a charged object will experience a force
    1. direction is given by the direction of the resultant force acting on a positive unit test charge at that point in the field
      1. positive to negative
  2. Force per unit charge: E = F/q
  3. Lines never cross
  4. enter and leave charged surfaces at right angles
  5. more lines means stronger field
  6. Uniform of parallel plates
    1. Force on test charge is constant and independant of position
5. Conductors and Insulators
  1. Conductors
    1. allow movement of electric charge
      1. electrons are loosely bound to individual nuclei
    2. metals are good due to nature of bonding
      1. regular lattice array of nuclei surrounded by free-roaming 'sea' of electrons; charge is free to move/spread over entire body
    3. distribute charges evenly across body
  2. Insulators
    1. allow little/no movement of electric charge
      1. electrons are fixed to individual nuclei
    2. localise any charge placed on body
Electric Circuits
1. Parallel
  1. voltage is the same across each component
  2. sum of currents across all components is equal to the current supplied by source
  3. resistance of components wired in parallel is less than resistance of individual components
  4. A break in one loop will not stop current flow in another loop
2. Series
  1. current is the same through all components
  2. sum of voltages across all components is equal to the voltage source
  3. resistance of components wired in circuit is greater than resistance of individual components
  4. A break in the circuit stops current flow
Current Electricity
1. Work and Energy
  1. Work = force applied over distance
    1. Work is done whenever a charge q is moved to a poin in space at distance d from the charge Q
    2. W = qEd
  2. Change in Electrical Potential Energy (EPE) is equal to the negative of the work DONE BY THE FIELD in moving a charge q from A to B
    1. ∆EPE = -qEd
    2. When work is done AGAINST the field, W is negative hence ∆EPE is positive
    3. EPE ∝ KE
      1. As Kinetic Energy increses, Electrical Potential Energy decreases accordingly
2. Electrical Potential (V)
  1. Electrical Potential Energy per unit charge
    1. V = ∆EPE/q V=-Ed
  2. Only differences are measurable
  3. Positive charge moves from high potential to low potential
3. Voltage (V)
  1. Potential difference between two points
  2. units are Volts (V) (joules per coulomb)
    1. measured using Voltmeter
4. Current (I)
  1. Rate of flow of electric charge
  2. unit is Ampere (A) (coulombs per second)
    1. measured using Ammeter
  3. Concentional current flows from positive to negative. This is a historical convention, and does not in fact represent the actual movement of particles (electrons flow from negative to positive)
5. Resistance (R)
  1. A measure of a component's opposition to the flow of electric charge
  2. unit is Ohms (Ω)
  3. A resistor impedes the flow of a current by converting EPE to heat as electrons collide with the cations within
  4. A voltmeter has a very high resistance, so as to divert minimal current away from a component
  5. An Ammeter has a very low resistance, in order to maintain the full current flowing in the circuit.
6. Ohm's Law: V=IR

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Electric Circuits

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	Created by Ursula Brown almost 10 years ago

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